1. Field of the Invention
Exemplary embodiments of the present invention relate to a transmission method and control technology for protecting transmit opportunities in a wireless LAN.
2. Description of Related Art
IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g standards for wireless LANs can support a transmission rate of 54 Mbps. Such IEEE 802.11a/g standards are designed to use a single antenna and a bandwidth of 20 MHz. In contrast, IEEE 802.11n standards can improve the transmission rate by using multi-antenna technology, i.e. use of a plurality of antennas, and bandwidth extension transmission technology, i.e. use of 40 MHz bandwidth. To be specific, IEEE 802.11n standards can support up to four antennas and can support a maximum of 600 Mbps physical layer transmission rate by applying a 64-QAM modulation scheme, a 5/6 code rate, and a short guard interval mode in a bandwidth of 40 MHz. In the MAC layer, furthermore, aggregation technology and BA (Block Acknowledge) technology are employed so that a throughput corresponding to about 86% of the physical layer transmission rate is obtained theoretically.
An important issue requiring consideration together with the history of establishment of wireless LAN standards is the backward compatibility problem. This means that, even in a network where IEEE 802.11a/g standard equipment and IEEE 802.11n standard equipment coexist, channels should be occupied according to a wireless LAN protocol based on fair competition without causing signal collision. To this end, IEEE 802.11n standards employ an L-SIG TXOP protection (Legacy Signal Field Transmit Opportunity protection) scheme. Specifically, during a TXOP interval in which a HT (High Throughput) mode (which is another name of IEEE 802.11n) packet (11n mode packet) is transmitted, a legacy signal field is used to set time to transmit the HT-mode packet so that legacy mobile stations, which failed to obtain rights to occupy channels, are prevented from occupying channels, thereby avoiding signal collision due to legacy equipment.
Recently, IEEE 802.11ac is extensively discussing VHT (Very High Throughput) technology as the next-generation wireless LAN technology. The IEEE 802.11ac standards are supposed to employ multi-user multi-antenna technology and multi-channel technology, and to have a longer period of time during which an access point and at least one mobile station occupy a channel. In summary, consideration of the above-mentioned problems based on such technical background reveals limitations of conventional TXOP protection methods using legacy signal fields.